The present invention relates to a band pass filter, and particularly, to a compact band pass filter that can be fabricated at low cost.
In general, a dielectric block having through holes passing from one surface to the opposite surface and all of whose surfaces except said one surface are metallized is used as a band pass filter. The through holes formed on the dielectric block work as resonators for the high frequency signal. The band pass filter is formed by adding capacitance and so forth to the resonators.
Many proposals have been made regarding methods for adding capacitance and so forth to the resonators constituted by the through holes.
According to one such method the dielectric block with the through holes is mounted on a substrate and the capacitors etc. are added to the substrate as separate components to form the band pass filter circuit. This method has the advantage that complex processing of the dielectric block is not required but has the disadvantage that the overall circuit size is enlarged because numerous components are used. The method is therefore not suitable for application to equipment that requires miniaturization, such as mobile phones.
According to another proposed method, conductive patterns that work as capacitors etc. are formed on said one surface of the dielectric block by screen-printing to form the band pass filter circuit. This method has the advantage that overall circuit size can be reduced because no capacitors etc. are added as different components, but has the disadvantage that it is extremely difficult to form the conductive patterns. Specifically, although the dielectric block should be stood and fixed with said one surface facing upward in order to print the conductive patterns on said one surface of the dielectric block, this is a very unstable state because the dimensions of said one surface and the opposite surface are small compared with the other surfaces. Further, high patterning accuracy is required for forming the conductive patterns on said one surface of the dielectric block but it is very difficult to form the conductive patterns accurately by the screen-printing in such an unstable state. Moreover, in certain types of the band pass filters, said opposite surface of the dielectric block is not flat but has a step. In this case it is extremely difficult to make the dielectric block stand with said one surface facing upward. For this reason, it is difficult to form the conductive patterns on said one surface of the dielectric block accurately and the fabricating cost increases. Another problem is that it is difficult to obtain large capacitance etc. by only forming the conductive patterns which work as capacitors on said one surface of the dielectric block.
In still another method, grooves or cavities are formed on said one surface of the dielectric block to form the band pass filter circuit by intentional disruption of the electromagnetic field balance. Like the method explained earlier, this method has the advantage that overall circuit size can be reduced but has the disadvantage that it increases fabricating cost because the conductive pattern must be formed on said one surface having the grooves or cavities by screen-printing with the dielectric block maintained with said one surface facing upward.
In still another method, proposed in Japanese Patent Laid Open No. 11-27006, a multilayered circuit member is added to said one surface of the dielectric block. This method has the advantage that it is easy to fabricate the dielectric block because no special processing for forming conductive patterns, cavities or the like on the dielectric block is required but has the disadvantage of high fabricating cost because the fabrication of the multilayered circuit member is complex, i.e., it has to be fabricated by laminating a number of dielectric layers each having a predetermined conductive pattern and through holes.
As pointed out above, the conventional methods for adding the capacitance and so forth to the resonators formed by the dielectric block having through holes encounter such problems as that overall circuit size is enlarged owing to the formation of numerous components or that the fabricating cost increases because it is difficult to fabricate the dielectric block or the multilayered circuit member to be added thereon.
It is therefore an object of the present invention to provide a compact band pass filter that can be fabricated at low cost.
The above and other objects of the present invention can be accomplished by a band pass filter comprising a dielectric block having through holes formed from one surface thereof to another surface opposite the one surface and a single layered dielectric plate having a front surface and a back surface joined to the dielectric block such that the back surface faces the one surface of the dielectric block, the dielectric block having metallizations formed on all surfaces except the one surface, and the dielectric plate having metallizations of predetermined patterns formed on the front and the back surfaces.
According to the present invention, a compact band pass filter that can be fabricated at low cost can be provided because it is formed by joining the single layered dielectric plate to the dielectric block. Further, since it is not necessary to form metallization on the one surface of the dielectric block, the dielectric block can be formed easily. Moreover, since metallizing on the front and back surfaces of the dielectric plate can be preformed in a stable state, the dielectric plate can be also formed easily.
In a preferred aspect of the present invention, the metallizations formed on the front and the back surfaces of the dielectric plate respectively form the one electrodes and the other electrodes of capacitors.
According to this preferred aspect of the present invention, capacitance can be added easily to the dielectric block because the metallizations formed on the opposite surfaces of the single layered dielectric plate form a capacitor.
In a further preferred aspect of the present invention, the metallizations formed on the back surface of the dielectric plate are in contact with the metallizations formed on inner walls of the through holes.
In a further preferred aspect of the present invention, the dielectric plate has cavities formed on the back surface thereof and the metallizations are formed on inner walls of the cavities.
According to this preferred aspect of the present invention, because the cavities are formed on the back surface of the dielectric plate, various characteristic can be given to the band pass filter by varying the shape of the cavities. Further, in the case where capacitors are formed by the metallizations formed on the inner walls of the cavities and metallizations formed on the front surface of the dielectric plate, large capacitance can be obtained.
In a further preferred aspect of the present invention, the dielectric plate has metallized projections projecting from the cavities, the metallizations formed on the projections being in contact with the metallizations formed on inner walls of the through holes.
In a further preferred aspect of the present invention, the through holes include at least a first through hole and a second through hole, the metallizations formed on the front surface of the dielectric plate include at least a first pattern and a second pattern, and the metallizations formed on the back surface of the dielectric plate include at least a third pattern and a fourth pattern, a first capacitor being formed by the first and third patterns, a second capacitor being formed by the second and fourth patterns, and the third and fourth patterns being in contact with the metallizations formed on inner walls of the first and second through holes, respectively.
In a further preferred aspect of the present invention, the first and second patterns are an input terminal and an output terminal, respectively.
In a further preferred aspect of the present invention, the dielectric block and the dielectric plate are made from the same material.
In a further preferred aspect of the present invention, the dielectric block and the dielectric plate are fixed by solder.
In a further preferred aspect of the present invention, every metallization is made of a conductive paste, and the dielectric block and the dielectric plate are fixed by sintering the conductive paste with the dielectric block and the dielectric plate joined.
According to this preferred aspect of the present invention, because the dielectric block and the dielectric plate are fixed by sintering the conductive paste, no solder is required. Therefore, lead (Pb) free soldering that should be performed at higher temperature than ordinary can be applied when the band pass filter is mounted.
In a further preferred aspect of the present invention, the back surface of the dielectric plate has a different area from that of the one surface of the dielectric block.